The recent advancements in automatic guided vehicle technology has resulted in vehicles which are capable traversing the floor of a warehouse, machine shop and the like without the need for a fixed floor mounted guidepath such as; guidance wires, stripes and the like. Automatic guided vehicles of this type are referred to by some as self guided vehicles (SGV). These vehicles are equipped with a programmable onboard computer which guides the vehicle in accordance with preprogrammed instructions and information provided to the computer by onboard sensors such as, wheel rotation and steer angle transducers, and the like. These sensors feedback information to the onboard computer which represents the actual operating conditions of the vehicle and enables the vehicle to perform to a limited degree, what is referred to as dead-reckoning. Dead-reckoning is the ability of the vehicle to continue to operate (travel) over a preprogrammed path without receiving any external input information for vehicle navigation purposes. During operation of the vehicle, conditions such as; wheel slip, tire wear, steering error and the like may exist which can affect the accuracy of the information provided by the sensors to the onboard computer. Therefore, in the long term, the vehicle will unknowingly deviate from the actual desired path of travel.
Vehicle navigation systems for SGV's have been provided to identify the actual location of the vehicle within the area of operation and deliver this information to the onboard computer so that the path of travel associated with dead-reckoning can be compared with the actual vehicle location and adjustments made to the vehicle position. The navigation system includes a laser scanner mounted on the vehicle. An example of an SGV having a laser scanner is disclosed in U.S. Pat. No. 4,647,784, dated Mar. 3, 1987, to Philip E. Stephens. The laser scanner delivers a light signal in essentially a horizontal sweeping plane and receives a reflection of the signal from targets located within the area of operation. The targets are located at preselected spaced apart distances within the area and at substantially equal heights from the surface so that the signal from the laser scanner can be reflected therefrom when in range. The laser scanner receives these signals and communicates with the onboard computer. The computer calculates the actual location of the using the method of triangulation, compares the actual location with the dead-reckoning position and maneuvers the vehicle to correct for this error. In normal circumstances the accuracy of vehicle position is extremely high. However, when the laser scanner is unable to read the targets due to obstructions in the path of the light beam, the vehicle guidance system, over long durations of time, will not be adequate to maintain the vehicle within acceptable tolerances along the desired path for the reasons discussed above.
In applications wherein rows of densely stacked boxes, cartons, and the like are present, the laser navigation signals have a high potential of being blocked. In such applications there is a need to enable the SGV to periodically verify its true location, in addition to the guidance signals associated with dead-reckoning, to insure that the SGV is located within acceptable tolerances of the desired path. Preferably, this can be achieved in an economical and efficient manner and without the need of complex devices.
The present invention is directed to overcoming one or more of the problems as set forth above.